The present invention relates to a connection method for connecting an assembly of a programmable logic controller to a bus.
The provision of leading contacts in an assembly, by means of which contacts the power supply contacts can make contact with a mounting rack before the other contacts of the assembly, is described in European Application No. 0 388 753. In this European Application, a resistor is arranged in the supply lines on the assembly, which resistor can be bridged by a switch. The switch is actuated only when a buffer capacitor arranged on the assembly is sufficiently charged.
A similar circuit arrangement is described in German Patent Number 39 42 480. Leading supply contacts and an electronic switch are present in this German Patent, too, the electronic switch coupling one of the supply lines directly to the load and the switch being closed only when a buffer capacitor has been sufficiently charged.
The abovementioned circuit solves the problem of impermissible feedback to the bus to which the assemblies are connected during insertion when the assemblies are functioning properly. If, however, the actual useful circuit is defective, in particular has a short circuit, it is possible that the power supply of an entire system collapses as a result of the insertion of the additional (defective) assembly.
A conventional connection method is described in International Patent Publication No. WO 93/15459. Waiting for a run-up time to elapse for the bus access circuit is described in a comparable technical context in European Patent Application No. 0 402 055.
The object of the present invention is to provide total system including a plurality of assemblies which is capable of running even when a defective assembly is connected to the total system during operation.
The object is achieved using the method and bus access circuit according to the present invention, with the following steps.
when the assembly is connected to the bus, the variable resistor is controlled to have a low resistance,
after the variable resistor has been controlled to have a low resistance there is a wait until a run-up time has elapsed, and
the variable resistor is controlled to have a high resistance again if a potential present at a test input of the evaluation circuit lies outside a predetermined value range after the run-up time has elapsed and is controlled to have a low resistance only after a connection time has elapsed.
In the case of the bus access circuit of the generic type, the problem is solved by the fact
that the evaluation circuit has a test input by means of which it is possible to acquire the potential present at the assembly,
the evaluation circuit is designed in such a way that it
controls the variable resistor to have a high resistance when an assembly is not connected the bus,
controls the variable resistor to have a low resistance when the assembly is connected to the bus,
waits for a run-up time to elapse after the variable resistor has been controlled to have a low resistance,
controls the variable resistor to have a high resistance again if the potential present at the test input lies outside a predetermined value range after the run-up time has elapsed controls the variable resistor to have a low resistance only after a connection time has elapsed.
The fact that the resistor is controlled to have a low resistance with the evaluation circuit only after a connection time has elapsed ensures that the assembly is reliably connected to the bus only when all the contacts of the assembly are connected to the corresponding bus contacts. Impermissible disturbances on the bus are thereby avoided.
The effect achieved by another embodiment according to the present invention is that the assembly is coupled smoothly to the bus. Feedback to the bus during the coupling of the assembly is thereby reduced even further.
The effect achieved by another embodiment according to the present invention is that when a defective assembly is connected, the signals transmitted via the bus are affected as little as possible.
The effect achieved by another embodiment according to the present invention is that the assembly is connected to the bus in terms of data technology only when it can in fact communicate via the bus. Disturbances to the bus traffic are thereby avoided.